Developers planning to build a $50 million baseball park on an old National Grid gas plant site in Malden will pay about $20 million over the course of a 40-year lease with the utility provider, according to a copy of the agreement obtained by the Globe.

The National Grid site takes up 6.4 acres, but the design for the park requires the acquisition of three other parcels totaling 2.7 acres. The lots are home to L&L Services at 11 Canal St., Spadafora’s Auto Parts at 129 Charles St., and Collex auto body repair at 124 Centre St.

On Monday, Bok described the other parcel owners as “fierce” negotiators who were previously unwilling to engage in serious talks until the National Grid parcel was purchased.

“We’re at stage 1A,” Bok said. “Now that we’re real, we’ll see what happens with that.”

Under the agreement with National Grid, Bok’s group would pay only $100 until a completion date on the park is finalized, or until a permanent certificate of occupancy is issued.

If that happens, the group would pay $100,000 in its first year of operation; $200,000 in its second; $325,000 in its third; and then the previous year’s rate plus a 2.5 percent increase each year after, according to the agreement.

The lease would run 40 years, with the developer having an opportunity to renew the lease for 10 years for the following four decades.

Coakley had to sign off on the lease because the site — which has been a gas plant since the 19th century — contains contaminents and requires environmental remediation. An agreement has been worked out with National Grid, Coakley said, but its details were not available Monday. The site has been vacant since the utility moved to another site in Malden earlier this year.

The terms of the lease had previously been approved by both parties and the Malden Redevelopment Authority in November, but the financial figures had not released because Coakley had not yet approved the deal.

Bok said the park — located in close proximity to the Malden Center Station on the MBTA Orange Line — is expected to produce about 125 part-time jobs during the six-month season, along with about 25 permanent, full-time positions. In addition to hosting about 70 games per year, the park also would host concerts and perhaps graduations, Bok said.

The Atlantic League currently has eight teams, which averaged about 4,400 in attendance last year, according to figures from the league.

Plans are in the works to host city-run public forums on the proposed project and conduct citizen surveys in coming weeks, Christenson said Monday. It would eventually have to go through the zoning approval process and meet other city requirements.

Before signing the commemorative bats, Christenson said the project was viewed as an economic driver for the city.

“I believe this project has the potential to not only revitalize this site but serve as an economic catalyst for the entire downtown,” the mayor said.